A hydrogen fuel cell is an electrochemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives a fuel such as hydrogen gas and the cathode receives an oxidant such as oxygen or air. Several fuel cells are typically combined in a fuel cell stack to generate a desired amount of power. A typical fuel cell stack for a vehicle may include several hundred individual cells. An illustrative fuel cell stack is disclosed in commonly owned U.S. Patent Application Publication No. 2004/0209150, hereby incorporated herein by reference in its entirety.
The fuel cell stack typically includes a wet end adapted to receive the fuel, oxidizer, and cooling fluids, and a dry end having an insulation end plate unit. The basic process employed by a fuel cell is efficient, substantially pollution-free, quiet, free from moving parts (other than an air compressor, cooling fans, pumps and actuators), and may be constructed to leave only heat and water as by-products. The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells depending upon the context in which it is used. The plurality of cells is typically bundled together and arranged to form a stack. The plurality of cells is commonly arranged in electrical series. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a desired energy output level. The stacks of varying sizes provide flexibility of design for different applications.
One type of fuel cell is a proton exchange membrane (PEM) fuel cell. The basic components of a PEM fuel cell are two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin catalyst layer. The electrodes, catalyst, and membrane together form a membrane electrode assembly (MEA).
In a typical PEM fuel cell, the MEA is sandwiched between “anode” and “cathode” diffusion media (DM) or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper, for example. The DM serves as the primary current collectors for the anode and cathode as well as provides mechanical support for the MEA. The DM and MEA are pressed between a pair of electrically conductive plates which serve as secondary current collectors for collecting the current from the primary current collectors. The plates conduct current between adjacent cells internally of the stack (in the case of bipolar plates) and conduct current externally of the stack (in the case of monopolar plates at the end of the stack).
The secondary current collector plates each contain at least one active region that distributes the gaseous reactants over the major faces of the anode and cathode. These active regions, also known as flow fields, typically include a plurality of lands which engage the primary current collector and define a plurality of grooves or flow channels therebetween. The channels supply the hydrogen and the oxygen to the electrodes on either side of the PEM. In particular, the hydrogen flows through the channels to the anode where the catalyst promotes separation into protons and electrons. On the opposite side of the PEM, the oxygen flows through the channels to the cathode where the oxygen attracts the hydrogen protons through the PEM. The electrons are captured as useful energy through an external circuit and are combined with the protons and oxygen to produce water vapor at the cathode side.
The polarity of an individual fuel cell within the fuel cell stack can be reversed if the stack load attempts to draw more electrical current from the stack than the cell can generate. Because the cells are electrically coupled in series, a low performing cell may experience cell overload if the remaining cells are able to supply the stack load. Under these operating conditions, the cathode side of the low performing cell becomes more negative than the anode side of the low performing cell bipolar plate, causing a reversal of polarity.
The polarity of individual fuel cells is susceptible to reversal during startup and shutdown. In particular, during the startup of the fuel cell, the anode flow field is substantially free of hydrogen. The hydrogen enters the anode flow field at the inlet and over a transitional time period the entire flow field fills with hydrogen. There is an insufficient supply of hydrogen in the anode flow field during the transitional time period to provide the required electrical output. During a shutdown period, there is a similar transitional period when the flow field is purged of hydrogen, and may not have adequate hydrogen to provide the desired electrical output. In the absence of the hydrogen to oxidize, the fuel cell will oxidize the catalyst disposed on the electrodes. The oxidation causes a degradation of the catalyst that reduces the ability of the fuel cell to produce an electrical current. The cumulative degradation of the catalyst during startup and shutdown eventually reduces the electrical output to such a level where the fuel cell stack must be rebuilt or replaced.
It is desirable to produce a fuel cell that militates against a polarity reversal of the fuel cell and minimizes a catalyst degradation caused by the polarity reversal of the fuel cell.